1. Field of the Invention
The present invention relates to a head position control method, head position-control device and disk device, and more particularly to a head position control method, head position control device and disk device for suppressing the position shift caused by a disturbance (NRRO) using observer control.
2. Description of the Related Art
It is very important for a disk device, such as a magnetic disk device or optical disk device, to accurately position the head on a target track in order to improve recording density.
For this positioning control, it is known that a disturbance influences the positioning accuracy. To suppress this disturbance by a control system, control systems in FIG. 26 to FIG. 28 are known conventionally. In the first prior art in FIG. 26, a position error ‘e’ between a target position ‘r’ and a current position ‘y’ of a plant 108 is computed by a computing block 100, and is input to a controller 102, and the controller 102 computes a control quantity to decrease the position error ‘e’. A filter 104 having a reverse characteristic form of a notch filter is added to this feedback control system for driving the plant 108 in parallel, to suppress a component (RRO component) near a specific frequency of the position error (see U.S. Pat. No. 6,487,028B1).
In the second prior art, a filter 104 is installed in series with the controller 102 in a feedback-loop in FIG. 26, as shown in FIG. 27, so as to suppress a component near a specific frequency of a control quantity of the controller 102 (see R. J. Bickel and M. Tomizuka: “Disturbance observer-based hybrid impedance control”, (Proceedings of the American Control Conference, 1995, pp. 729-733)).
In the third prior art, a value when a current position ‘y’ is divided by a transfer function P of a plant 108 in a block 110, that is a difference between a second order differential value of a position error and a command current value from a computing block 106, is determined by a computing block 112 as a disturbance observer in a feedback loop in FIG. 26, and is fed back to the computing block 106 via a band pass filter (also called a Q filter) 114, as shown in FIG. 28 (see R. J. Bickel and M. Tomizuka: “Disturbance observer-based hybrid impedance control”, (Proceedings of the American Control Conference, 1995, pp. 729-733)).
Also in order to handle an eccentricity of a disk, which is the periodic disturbance (RRO), a method for correcting the eccentricity using an eccentricity estimation observer has been proposed (e.g. Japanese Patent Application Laid-Open No. H7-50075 or Japanese Patent Application Laid-Open No. 2000-21104).
Such an eccentricity estimation observer calculates a control value of an actuator from an error between an actual position error and estimated position error, using state estimation gains A, B, C, F and L, and calculates state quantity (position, velocity, bias value, eccentricity) of the next sample.
Here the estimated gain L consists of an estimated position gain L1, estimated velocity gain L2, estimated bias gain L3 and estimated eccentricity gains L4 and L5. And L1, L2 and L3 show characteristics of the controller itself, and L4 and L5 show response characteristics to eccentricity, which is a periodic disturbance.